Advances in Phosphors for Light-emitting...

rXXXX American Chemical Society 1268 dx.doi.org/10.1021/jz2002452 | J. Phys. Chem. Lett. 2011, 2, 1268–1277

PERSPECTIVE

pubs.acs.org/JPCL

Advances in Phosphors for Light-emitting DiodesChun Che Lin and Ru-Shi Liu*

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan

Following an increasing awareness of climate change andenvironmental issues, people are looking for alternatives to

fossil fuels as energy sources that do not emit carbon dioxide.1�3

White light-emitting diodes (WLEDs) are extensively used andare very important as they significantly reduce global powerrequirements and the use of fossil fuels.4 They have attractedsubstantial attention owing to their extraordinary luminousefficiency, low power consumption, reliability, and environmen-tal friendliness. The quest for phosphors for lighting is one of themost important and urgent challenges to be met by advancedscience and high technology, and novel and acceptable fluores-cent-material-based solid-state lighting (SSL)must be developed.5

As presented in Tables 1 and 2, the intensity, width, durability,and thermal quenching of commercial phosphors for light-emitting diodes (LEDs) suggest two exciting approaches ofUV-LED chip (with the wavelength of 380�420 nm) andblue-LED chip (with the wavelength of 450�480 nm) foraccelerating this development. One involves mixing the emis-sions from red, green, and blue (RGB) phosphors with a UV-LED chip, as in a device with the schematic structure that isshown in the left inset in Figure 1A. The disadvantages of such adevice are low efficiency of the red phosphors (due to the largeStokes shift) and the need for complex coating technology (e.g.,problems of sedimentation and uniformity distribution of phos-phors in silicon resin). Furthermore, mixing powders and findinghigh-efficiency compounds are more difficult for this type device.However, there are several benefits, including high color render-ing index, high luminous efficiency, and stable light color that arealmost independent of the changed current. According to therelevant literature,6,7 all such devices use inorganic phosphorsthat are mixed with quantum dots, which act as an alternative to adown-converter in WLEDs. Therefore, we propose a novelmixture of variously colored quantum dots (InP) and siliconresin as a color-converting material, which can be applied to aUV-LED chip, as displayed in the right inset in Figure 1A. In the

case of nontoxic InP QDs,8 the full color emission wavelengthscan be easily adjusted by controlling the particle size (quantumconfinement effect), and such QDs can be dispersed uniformly insilicon resin. This fact can perhaps be exploited to solve theproblems of the efficiency and coating technology of UV-LEDdevices.

The other method for lighting devices involves Ce3þ-dopedyttrium aluminum garnet (YAG/Ce)-based blue-LED chips,which has been commercialized. The left inset in Figure 1Bpresents such a device. It can offer advantages such as easyfabrication and low cost. Nevertheless, the unstable light colorresults in the halo phenomenon under the different outputcurrent. The light from YAG/Ce-based WLEDs is colder andbluer than that from a traditional incandescent lamp. Its colorrendering is poor owing to the red deficiency of the yellowphosphor. This problem has attracted the attention of manyresearchers, who have sought to improve the color renderingproperty of phosphor-converted (pc) WLEDs. Core�shells orcore�multishells of CdSe-based core QDs have been widelyutilized to improve the color rendering index (CRI) ofWLEDs.9,10

The right inset in Figure 1B shows a new combination of broademission Lu3Al5O12/Ce

3þ (LuAG/Ce) yellow�green phos-phors and highly efficient red InP QDs. Moreover, Figure 1Cexhibits the efficiency curves of different LED chips plotted asa function of correlated color temperature (CCT). To realizea low CCT of a blue-LED chip, a very thick phosphor layermust be formed to reduce the transparency of blue light. Avery thick phosphor layer limits brightness because it pre-vents fluorescent light from traveling from the LED side tothe front of the device. If a violet LED is used to yield lowCCT, then the phosphor layer can be optimized to pass light

Received: February 23, 2011Accepted: May 6, 2011

ABSTRACT: Light-emitting diodes (LEDs) are excellent candidates forgeneral lighting because of their rapidly improving efficiency, durability, andreliability, their usability in products of various sizes, and their environmentallyfriendly constituents. Effective lighting devices can be realized by combiningone or more phosphor materials with chips. Accordingly, it is very importantthat the architecture of phosphors be developed. Although numerous phos-phors have been proposed in the past several years, the range of phosphors thatare suitable for LEDs is limited. This work describes recent progress in ourunderstanding of the prescription, morphology, structure, spectrum, andpackaging of such phosphors. It suggests avenues for further developmentand the scientific challenges that must be overcome before phosphors can bepractically applied in LEDs.

1269 dx.doi.org/10.1021/jz2002452 |J. Phys. Chem. Lett. 2011, 2, 1268–1277

The Journal of Physical Chemistry Letters PERSPECTIVE

from the LED side to the front of the device. The low CCTcan be obtained just by adjusting the mixing ratio of the RGBphosphors even without adjusting the thickness of the phos-phor layer. As expected from the device scheme in Figure 1,semiconductor nanocrystals or QDs are a promising alter-native to down-converting materials in WLEDs because oftheir attractive properties, including size-tunable opticalcharacteristics, broad absorption spectrum, narrow emissionband, high quantum yields, and low light scattering.11,12 Thecriteria that must be applied and the approach that must betaken to enable inorganic phosphors to be used in LEDs are ofconcern.

On the basis of the above mentioned, rare-earth ions, likeEu2þ, Ce3þ, Tb3þ, and Sm3þ, which work as luminescencecenters in most inorganic phosphors, tend to be very expensive.Moreover, a large number of productions were usually synthe-sized under high temperatures, high pressures, or reducingconditions. The development of available approaches for fluo-rescent materials without using rare-earth ions at relatively lowtemperature under air atmosphere is agreeable to apply in thefuture lighting. We will introduce some non-rare-earth-basedphosphors in this Perspective. Notably, the blue-color-emittingGaZnON materials have not yet been reported in previousliterature. This Perspective highlights some characteristics of

Table 1. Examples of WLEDs that Incorporate UV-LEDs Excitable Phosphors. (O: Good; 4: Medium; �: Bad)

emission characteristics

LED phosphor chemical composition intensity width durability thermal quenching

violet LED blue phosphor (Sr,Ca,Ba,Mg)10(PO4)6Cl2/Eu O narrow O 4

(Ba,Sr)MgAl10O17/Eu O middle O O

(Sr,Ba)3MgSi2O8/Eu O narrow 4 4

green phosphor SrGa2S4/Eu O middle � �β-sialon/Eu O middle O O

SrSi2O2N2/Eu O middle O O

Ba3Si6O12N2/Eu O middle O O

BaMgAl10O17/Eu,Mn O narrow O O

SrAl2O4/Eu 4 broad 4 4

red phosphor (Sr,Ca)S/Eu O broad � �(Ca,Sr)2Si5N8/Eu 4 broad 4 4

CaAlSiN3/Eu O broad O O

La2O2S/Eu 4 narrow 4 4

3.5MgO 3 0.5MgF2 3GeO2/Mn 4 narrow O O

(Sr,Ca,Ba,Mg)10(PO4)6Cl2/Eu,Mn 4 broad O O

Ba3MgSi2O8/Eu,Mn O broad 4 4

Table 2. Examples of WLEDs that Incorporate Blue-LEDs Excitable Phosphors. (O: Good; 4: Medium; �: Bad)

emission characteristics

LED phosphor chemical composition intensity width durability thermal quenching

blue LED green phosphor Y3(Al,Ga)5O12/Ce 4 broad O 4

SrGa2S4/Eu O middle � �(Ba,Sr)2SiO4/Eu O middle 4 4

Ca3Sc2Si3O12/Ce O broad O O

CaSc2O4/Ce O broad O O

β-sialon/Eu O middle O O

(Sr,Ba)Si2O2N2/Eu O middle 4 O

Ba3Si6O12N2/Eu O middle O O

yellow phosphor (Y,Gd)3Al5O12/Ce O broad O 4

Tb3Al5O12/Ce 4 broad O 4

CaGa2S4/Eu O middle � �(Sr,Ca,Ba)2SiO4/Eu O broad O 4

Ca-R-sialon/Eu O middle O O

red phosphor (Sr,Ca)S/Eu O broad � �(Ca,Sr)2Si5N8/Eu O broad 4 4

CaAlSiN3/Eu O broad O O

(Sr,Ba)3SiO5/Eu O broad � O

K2SiF6/Mn O narrow O O



phosphors and provides an overview of the subject area forWLEDs in the future.

YVO4/Bi3þ,Eu3þ: Spectral Characteristics and Formula. Some

intelligent algorithms, such as genetic algorithms, Monte Carlotechniques, simulated annealing, and artificial neural networksalgorithms, can be used to improve the efficiency of new materialsand to optimize the objective compounds.13�16 Recently, Sohnet al.17 employed solid-state combinatorial chemistry to synthesizeAE2Si5N8/Eu

2þ (AE = alkaline earth elements) phosphors andanalyze their composition. Liu et al.18 screened (YxLu1�x�y)3-Al5O12/Ce3y green�yellow phosphors using the same method

(combi-chem). Figure 2A schematically depicts the developeddrop-on-demand inkjet delivery system. A driving circuit and acomputer that controls the motor drive the piezoelectric inkjetheads and the X�Y stage of a substrate. The emission spectra ofthe samples in the library were measured using an automaticsystem that was comprised of a Hg Lamp, a portable optical fiberspectrometer (Ocean Optics, Inc., model SD2000), and an X�Ystage. Figure 2B displays the composition map of theY1�s�tVO4/Bi

3þs,Eu

3þt combinatorial library and a luminescent

photograph obtained under 365 nm excitation. The first columnclearly reveals faint red emission from Eu3þ at various concen-trations (t = 0�0.060) without Bi3þ codoping (s = 0). The firstrow clearly reveals that without Eu3þ codoping (t = 0), the stronggreen emission of Bi3þ increases with Bi3þ concentration froms = 0 to 0.050; the increase becomes negligible when the Bi3þ

content exceeds s = 0.025. Furthermore, the emission colors in allother rows (t = 0.005, 0.015, 0.030, 0.045, and 0.060) remainalmost constant with different Bi3þ concentrations, but thebrightness increases with Bi3þ concentration from s = 0.005 to0.040 and then declines as the Bi3þ concentration increases

Figure 1. Structures for generating white light from fluorescent materials, based on (A) UV chip and (B) blue chip. (C) Efficiency of LED chips as afunction of correlated color temperature.

Combinatorial chemistry is used tooptimize novel phosphors for ap-plication in light-emitting diodes.



further. These results suggest that Eu3þ cannot emit efficientlywithout Bi3þ codoping under 365 nm excitation but that excessiveEu3þ codoping severely reduces Bi3þ luminescence. Figure 2Cshows the emission spectra of the samples at t = 0.005 and s =0�0.050 in the combinatorial library under 365 nm excitation.The emission lines at 592, 618, 650, and 702 nm are attributed tothe 5D0 f

7FJ (J = 1, 2, 3, 4) transitions of Eu3þ, and the broademission band with a peak at 545 nm is assigned to the 3P1f

1S0transition of Bi3þ.16 The emission intensity of Eu3þ increasescontinuously with Bi3þ concentration from s = 0 to 0.050,

indicating the transfer of energy between Bi3þ and Eu3þ. Mean-while, the emission intensity of Bi3þ increases with concentra-tion. The inset in Figure 2C plots the variation of the relativeheight of the Bi3þ emission peaks at 545 nm and the Eu3þ

emission peaks at 618 nm with Bi3þ concentration. The rates ofincrease of Eu3þ and Bi3þ emissions are similar to each other ats = 0�0.025, but the rate of increase of Eu3þ emission exceedsthat of Bi3þ at s = 0.025�0.050. The energy transfer from Bi3þ toEu3þ does not influence Bi3þ emission if the concentration ofBi3þ does not exceed s = 0.040 and that of Eu3þ is less than

Figure 2. (A) Drop-on-demand inkjet delivery system. (B) Composition map of the (Y1�s�tBisEut)VO4 combinatorial library and luminescentphotograph under 365 nm excitation. (C) Emission spectra of samples with s = 0�0.050 and t = 0.005 in the (Y1�s�tBisEut)VO4 combinatorial libraryunder 365 nm excitation. (D) Emission spectra of samples with s = 0.040 and t = 0�0.060 in the (Y1�s�tBisEut)VO4 combinatorial library under 365 nmexcitation. (E) Emission spectrum of a warm-white LED lamp. (Adapted from ref 16.)



t = 0.005. However, Bi3þ emission is clearly degraded by theshortening of the mean distance between Bi3þ and Eu3þ, andEu3þ then efficiently captures energy from Bi3þ. The results inFigure 2D agree well with those in Figure 2C when the emissionof Bi3þ declines gradually as the Eu3þ content increases from t =0 to 0.060, indicating that the shortening of the distance betweenBi3þ and Eu3þ results in the transfer of energy from Bi3þ to Eu3þ

to an extent that increases gradually with Eu3þ concentration.The above results concerning the photoluminescence that is

excited by UV light (365 nm) are consistent with the results ofconventional-scale synthesis, revealing that the combinatorialchemistry technique is fast, reliable, and reproducible. Therefore,a novel prescription of phosphors can be found rapidly andexhaustively by using a combinatorial chemistry approach. Apractical white LED lamp was fabricated by coating a NUV-LEDchip with commercially available Sr3MgSi2O8/Eu

2þ (3128) bluephosphor and self-optimized (Y0.956Bi0.040Eu0.004)VO4 phos-phor. Figure 2E presents the emission spectrum of the preparedLED lamp, with a Ra of up to 90.3.

BaMgAl10O17/Eu,Mn: Moisture Measurement and Morphology.The flux plays an important role in improving crystallinity, promot-ing grain growth. It is even involved in controlling the morphologyand size of a phosphor.19 On the basis of previous investigations,the addition of a flux enhances the luminescence intensity andreduces the reaction temperature of fluorescent materials. Thestability of the moisture content and the emission intensity ofBaMgAl10O17/Eu,Mn phosphors vary with various fluxes, suchas AlF3, BaF2, and H3BO3. Figure 3A shows the PL spectra of(Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. Thespectra have similar shapes but various emission intensities.

Figure 3. (A) PL spectra of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes (excited at 370 nm). (B) Scanning electron micrographs of(Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. (C) Conductivity of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes as a function oftime. (D) PL spectra of as-received and hydrated (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with AlF3 and H3BO3 fluxes (excited at 370 nm). (Adapted fromrefs 21.)

Fluxes considerably improve thestability of phosphors against

moisture.



The fluxes significantly enhance the luminescence intensity andcolor saturation. Accordingly, the phosphor that is synthesizedwith AlF3 has a higher relative emission intensity (116%) andcolor purity than commercial BAM phosphor. The involvementof fluxes in the mechanism of BAM particle growth has not beenelucidated in detail. However, fluxes are broadly agreed to be

compounds of alkali or alkaline earth metals with low meltingpoints; when fluxes melt, the surface tension of the liquid helpsparticles coagulate and facilitates their sliding and rotation,providing more opportunities for particle�particle contact andpromoting particle growth. Recent work has established thatwhen fluorides are used as fluxes, BAM particle growth may

Figure 4. (A) Calculated density of states (DOS) of (a) pure KSrPO4 and KSr1�xPO4/Eux (x = 3.125%) in the (b) non-spin-polarized state,(c) spin-polarized state, and (d) spin-polarized state with on-site Coulomb interaction. (B) Majority spin orbital structure and possible mechanism ofelectronic transition in the KSrPO4/Eu system. (Adapted from ref 24.)



proceed by the following mechanism20

BaCO3 þMgOþ 4=3AlF3 f BaMgF4 þ 2=3Al2O3 þ CO2

BaMgF4 þ 17=3Al2O3 f BaMgAl10O17 þ 4=3AlF3

MgOþ 2BaF2 f BaMgF4 þ BaO

BaMgF4 þ 5Al2O3 þ 2BaO f BaMgAl10O17 þ 2BaF2

As revealed by these chemical equations, the fluoride reacts withsome of the raw material to produce BaMgF4 at approxi-mately 1000 �C, which melts at a temperatures of over 1000 �C.BaMgAl10O17 will begin to be created in BaMgF4 solutions atabout 1200 �C. At around 1600 �C, all of the BAM are created.

Figure 3B displays the SEM morphology of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with various fluxes. The widely dispersedBAM phosphor has a plate-shaped morphology because of itscrystallographic characteristics. A comparison with a phosphorprepared without flux demonstrates that the growth of particlesin the presence of flux is extensive; the flux improves their crys-tallinity, and the phosphor particles with a fluoride-based fluxhave a hexagonal surface morphology. A sample that is preparedby H3BO3 flux is semispherical and not smooth. The results alsoindicate that the morphology of phosphor particles affects theirluminous properties and stability.21 Mishra et al.22 observed thatmoisture can easily interact with the intermediate planes of thealuminates in a β-alumina structure, degrading the BAM. Chemi-cal stability is regarded as an important parameter in the applica-tion of phosphors in WLEDs, plasma display panels, and fluores-cent lamps. Figure 3C plots the conductivity of (Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 as a function of timewith various fluxes. Theconductivity increases with time because some ions of thephosphor host are dissolved in water. Specifically, the conductivityvaries only a little in the synthesis of BAM with AlF3 or BaF2fluoride-based fluxes, enhancing the stability againstmoisture. Thisresult is obtained because phosphor particles with a fluoride-basedflux, which have a highly crystalline hexagonal surfacemorphology,cannot easily interact with themoisture. However, the sample withH3BO3 as the flux has the lowest stability owing to the absence ofhighly crystalline particles and the smoothness of the surface.Figure 3D shows the PL spectra of the as-received and hydrated(Ba0.85Eu0.15)(Mg0.7Mn0.3)Al10O17 with AlF3 and H3BO3 asfluxes. The emission spectra indicate a reduction in the emissionintensity of both of the water-treated samples. The total integratedradiance of the hydrated samples with AlF3 and H3BO3 as fluxes is�9 and�22% less than that of fresh powder, respectively. Hence,synthesis with fluxes can enhance the stability against moisture;AlF3 outperforms BaF2 or H3BO3. Furthermore, the selection ofsuitable fluxes in the synthesis of any compounds can improve thestability against moisture. Chemical stability is an importantrequirement of phosphors for use in LEDs.

Chen et al.23 calculated the electronic structure of RMO4 (R =Y, Gd; M = P, V) and elucidated its physical properties and

interatomic interactions by employing density functional theory(DFT). Although various densities of states (DOSs) of undopedhosts have been reported, the DOS of doped phosphors had notbeen calculated before the KSrPO4/Eu system was developed.24

Figure 4 shows the DOS of, and a possiblemechanism of electrontransition in, the KSrPO4/Eu system. The results reveal that pureKSrPO4 has a direct band gap of approximately 5.09 eV at pointΓin (a) of Figure 4A. The band gap is shifted only slightly when the800 eV cutoff energy for the plane wave basis and the geometricaloptimization of the KSrPO4 are considered, thus suggesting thatthese parameters have only a weak effect on the electronicstructure of KSrPO4 systems. The reference energies are set tothe highest-energy electron-occupied state. The upper and lowerrows in (c) and (d) of Figure 4A concern the majority spin (spin-up) and minority spin (spin-down), respectively. The electronicstructure of KSrPO4 after it is doped with dilute Eu is presentedbelow; band gap values of around 1.3 (b) and 2.5 eV (c) areobtained for non-spin-polarized and spin-polarized electrons,respectively. With respect to the spin polarization and the on-siteCoulomb interaction of Eu 4f electrons, the majority spin Eu 4fstates are fully occupied, and the main peak of the Eu 5d statesappears at about 3.0 eV above the Fermi level in (d) of Figure 4A.Remarkably, the separation of 3.0 eV between the Eu 4f�5dmain

Figure 5. (A) Temperature-dependent emission spectra of Y3Al5O12/Ce3þ and CaAlSiN3/Eu

2þ obtained under 460 nm excitation. (B)Configuration coordinate diagram. (Adapted from ref 3.)

The mechanism of electronic tran-sition was confirmed by theoretical

calculations.



peaks, corresponding to the wavelength of 414 nm, is veryclose to the measured emission wavelength of 424 nm. There-fore, the calculations herein suggest a possible mechanism ofexcitation from Eu 4f to the conduction band (CB) via Eu 5d,followed by the main emission from Eu 5d to Eu 4f. On thebasis of the above discussion, Figure 4B presents a proposedmechanism of electron transition in the KSrPO4/Eu system.Initially, the Eu 4f f CB excitation pumps electrons to thedelocalized CB via Eu 5d. Then, nonradiative relaxation bringsthe electron to the main peak of Eu 5d close to the lower bandedge of the CB. Finally, photoemission may be associated withthe on-site Eu 5d�4f transition.

The thermal stability of phosphors that are used in WLEDsmust be understood. Figure 5A plots the relationship betweenthe emission intensity and the environmental temperature ofY3Al5O12/Ce

3þ and CaAlSiN3/Eu2þ, measured under 460 nm

excitation. The emission intensity of the two samples declined asthe temperature increased because the nonradioactive transition

from the excited states to the ground state increased by thecrossing point (F), as shown in the configurationally coordinatediagram in Figure 5B; this effect is called thermal quenching. As aresult, the emission peaks of YAG/Ce3þ were red-shifted from560 to 570 nm, which is explained with reference to the Varshiniequation for the temperature dependence of energy25

EðTÞ ¼ E0 � aT2

T þ b

where E(T) is the energy difference between the excited statesand the ground states at temperature T; E0 is the correspondingenergy difference at 0 K, and a and b are fitting parameters.Increasing the temperature reduces the transition energy and redshifts the emission peak. Additionally, the configurationallycoordinate (red line) is slightly right-shifted from that of theoriginal coordinate (black line), reducing the activation energy(ΔE = F00 to G). Sohn et al.26 found that a lower energy emissionof CaAlSiN3/Eu

2þ, which was attributed to an Al-rich localenvironment, made the polyhedron around the Eu2þ activatorsmaller, while a Si-rich local environment resulted in a largerpolyhedron and a higher energy emission. Consequently, theemission peaks of CaAlSiN3/Eu

2þ in Figure 5Awere blue-shiftedin a manner determined by the configuration coordinate (blueline), increasing the activation energy (ΔE = F0 to G).

Prospects for Phosphors in WLEDs. As described above, in thesearch for suitable phosphors, significant effort has been made todevelop highly efficient LED devices. Figure 6 presents thedeveloped scheme of phosphors for WLEDs. In recent years,several highly luminous YAG/Ce or silicate-based blue-LEDchips have been fabricated, but they have a low color-renderingindex and low thermal stability. Although nitride and phosphatephosphors have high thermal stability, they have very lowefficiency. The optimal mixture of Sr3MgSi2O8 (blue), LuAG(yellow�green), and CaAlSiN3 (red) was recently proposed foruse in a high-efficiency device. Additionally, considering the costof precursors and synthetic processes, non-rare-earth-basedphosphors maybe are excellent candidates for replacing dopedmaterials in the preparation of LEDs. Okuyama et al.27 havereported full-color-emitting BCNO phosphors, which are pro-duced by a one-step liquid process at relatively low temperature(800 �C). The peak positions of emission spectra shift from theUV (387 nm) to visible (571 nm) under different conditions,

The thermal stability of phosphorscan affect color coordinates oflighting for practical packaging.

Figure 6. Developed scheme of phosphors for WLEDs.

Figure 7. (A) Diffuse reflection spectra for Ga2O3, ZnO, and GaZnON. (B) Excitation (λem = 450 nm) and emission (λex = 390 nm) spectra forGaZnON.



including the polyethylene glycol/boron ratio, reaction tempera-ture, and heating time. Nanda et al.28 have successfully developednanorod Ga2O3 that been used as a multilight emitter forWLEDs.They also explained the possible growth mechanism and lumi-nescence mechanism for this compound. The bluish-greenemission was ascribed to the oxygen vacancy, while the nitrogenformed the red emission. Especially, the solid-solution GaZnONwas fabricated without doping rare-earth ions at considerably lowtemperatures (below 500 �C) under ambient atmospheric pres-sure. Figure 7A presents the reflection spectra of the precursorsGa2O3 and ZnO and the production of GaZnON. It clearlyindicates that the optical band gap was estimated to be 4.9 and 3.3eV for Ga2O3 and ZnO, respectively. The band gap of GaZnON,which has stronger absorption (380�450 nm) than others, wasmeasured to be 3.7 eV. Figure 7B shows the excitation andemission spectra of the GaZnON phosphor. The excitationwavelength, which is consistent with the reflection spectra, issuitable for the UV-chip- and blue-chip-based LEDs. The PLspectrum exhibits a pure blue emission band from 400 to 550 nmcentered at 450 nm ascribed to the oxygen vacancy and urea. Thechromaticity coordinate of this compound was found to be(0.1614, 0.1439), in the higher blue color purity region. We willperform more studies to verify the luminescence mechanism andthe distribution of different elements in the near future. Briefly,considerable effort will be devoted to the development of thenon-rare-earth-based phosphors for exercise in WLEDs. ThisPerspective highlights the development and tendency of phos-phors and provides some methods that can readily be scaled upfor industrial applications.

’AUTHOR INFORMATION

Corresponding Author*Tel:þ886-2-33661169. Fax:þ886-2-23636359. E-mail: [email protected].

’BIOGRAPHIES

Chun Che Lin received his B.S. degree in chemistry fromChung Yuan Christian University in 2005. He received his M.S.degree in chemistry from National Taiwan University in 2007.He is currently working on his Ph.D. in inorganic chemistry,focusing on the synthesis of fluorescent materials for particularapplications. His current research interests include synthesis ofphosphors and quantum dots for LEDs and bioapplications.

Ru-Shi Liu is a professor at the Department of Chemistry,National Taiwan University. He obtained two Ph.D. degrees inchemistry, one from National Tsing Hua University in 1990 andanother from the University of Cambridge in 1992. He worked atthe Industrial Technology Research Institute from 1983 to 1985.

’ACKNOWLEDGMENT

The authors would like to thank the National Science Councilof the Republic of China, Taiwan, for financially supporting thisresearch under Contracts NSC 97-2113-M-002-012-MY3, andNSC 97-3114-M-002-002.

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